1. Field of the Invention
This invention relates generally to lubricants and more particularly, to lubricant compositions and methods for their use in a run-flat tire system.
2. Description of the Related Art
Run-flat tire systems have been developed to provide tires for vehicles that can be operated in a deflated condition for a suitable distance at a desired speed. Such systems have provided significant benefits to vehicle operators including safety benefits that are realized when, for example, a vehicle can continue traveling with a deflated tire under conditions that are not safe for changing the tire at the side of a road. Thus, run-flat tire systems improve the safety of the vehicle by allowing the vehicle to continue to travel for a certain time until a suitable place for repairs can be found.
Run-flat systems having safety support rings are well known. U.S. Pat. No. 6,944,948 of Pompier, which has been assigned to Michelin Recherche et Technique, discloses such a system. Pompier discloses a run-flat tire system having a safety support ring consisting of a circular body adapted for fitting onto a wheel rim of a vehicle. The disclosed safety support ring is comprised of a vulcanized rubber mix and includes areas that are reinforced by, for example, metallic or textile wires or cables. While Pompier discloses a safety support ring made of a vulcanized rubber mix, the support rings are not so limited and may be made, for example, of plastics such as polyurethane.
As Pompier discloses, safety support rings are generally mounted on a wheel rim inside a tire to provide support for the crown of the tire when it is rolling at low or zero pressure. The safety support ring is meant to prevent direct contact between the tire and the wheel rim because such contact generally results in rapid deterioration of the tire.
To improve rolling under the condition of low or zero pressure, it is preferable to provide lubrication at the interface between the inside surface of the tire and the surface of the support ring. Such lubrication promotes extended durability in the friction zones that are subjected to relatively high temperatures due to the friction between the surfaces if no lubrication is provided.
One example of a lubricant composition that has been used for lubricating the interface between the inside surface of a tire and the surface of a safety support ring is disclosed in U.S. Pat. No. 6,750,181 of Salaun, et al, which has been assigned to Michelin Recherche et Technique. The lubricant composition disclosed by Salaun includes an aqueous or nonaqueous lubricating agent and a polysaccharide intended to thicken the lubricating agent. A preferred lubricating agent is disclosed as glycerin.
Salaun further discloses that the preferred lubricating composition has a viscosity of between 100,000 centipoise and 160,000 centipoise at 25° C. and at atmospheric pressure, measured in accordance with the “Brookfield” technique by means of a spindle designated “LV4.” Salaun further discloses that the polysaccharide used as a thickening agent makes it possible to impart a thixotropic nature to the lubricating composition.
As known to those having ordinary skill in the art, a thixotropic material is a pseudoplastic non-Newtonian fluid that, after undergoing shear thinning, has the potential to have its structure reformed when allowed to rest over a period of time. A pseudoplastic fluid has a viscosity that decreases as the applied shear rate increases and that decreases at a constant applied shear rate. The process of applying a shear rate to decrease viscosity is called shear thinning, Therefore, a thixotropic material that is a gel, for example, may become a free-flowing liquid when subjected to shear thinning but upon the cessation of the applied shear rate, the structure of the material may reform and convert back to a gel over time as the material rests.
Another example of a lubricant composition that has been used in a run-flat tire system is disclosed in the PCT patent application WO2004/069565 of Mauclin, et al., which has been assigned to Michelin Recherche et Technique. Mauclin discloses a lubricating composition that includes a lubricating agent and a thickening agent, the preferred lubricating agents being an alkene oxide polymer. The preferred thickening agent is disclosed as being fumed silica.
Generally, the lubricant compositions of the run-flat tire system may be applied to the surface of the safety support ring, the inside surface of the tire or both. The thickening agent added to the lubricant composition is intended to increase the viscosity of the lubricating agent so as to minimize the flowing of the lubricating agent due to its weight when the vehicle is at rest or is traveling with its tires inflated. If the lubricating agent does not remain in place, then the tire may become unbalanced and affect the smoothness of the ride.
Some lubricants having a thixotropic nature have had problems remaining in place after having been subjected to an intermittent shear that causes their viscosity to decrease, thereby turning the lubricant into a flowing fluid. Such intermittent shear may occur, for example, when the tire hits a bump in the road causing the tire to deflect so that the inner surface of the tire momentarily contacts the surface of the safety support ring, thereby imparting shear to the thixotropic lubricant. Such intermittent contact may also occur, for example, during tight cornering.
Therefore, even though the literature includes many different lubricants that are disclosed as being suitable for a run-flat tire system, there is still a need for an improved lubricant. There is a need for a lubricant that does not flow after undergoing intermittent shear caused, for example, by the tire hitting a bump in the road or being subjected to tight cornering.